The present invention relates broadly to a waste recycling and metal recovery method. More particularly, the present invention relates to a method for recycling metal-bearing hazardous wastes to recover valuable metals and metal oxides. In the best mode, slags remaining after metal recovery are used to produce mineral wool, and no waste results.
Public awareness of problems associated with the rapid depletion of the earth's natural resources and disposal of industrial wastes has greatly increased in recent times. Such awareness, together with increased economic pressures, the tightening of competition, and government regulation of wastes have forced industrial concerns to take measures to minimize waste. In response, the focus of scientific undertaking in some industries has turned toward recovery and reuse of all commercially useful byproducts of industrial processes.
In the past, little attention was directed to the preservation of limited mineral resources. It was generally deemed more feasible to mine metal ores and to simply dump rich metal-bearing wastes than to salvage usable metals from waste products. This was particularly true in the case of industrial wastes that contained hazardous or toxic materials.
Hazardous industrial wastes were typically "stabilized" and/or captured in some generally non-leachable form with a basic material such as lime or cement. It was required to bury the stabilized materials in designated hazardous waste landfills. One widely accepted disposal method was to incorporate hazardous waste products into a glass-like matrix called slag that was used as a substitute for a natural rock aggregate in cement or asphalt used for paving roads and the like. However, the EPA has declared that no material that is considered hazardous can be applied to the land in any form, whether or not it has been diluted, treated, or otherwise stabilized. Hence, the older disposal methods have fallen into disfavor. Alternative processes for treating wastes to produce environmentally "safe" products have been proposed.
For example, U.S. Pat. No. 4,432,666 issued to Frey on Feb. 21, 1984 describes a process for storing and dumping hazardous wastes. Rostoker, U.S. Pat. No. 4,793,933 issued Dec. 27, 1988 teaches a method for treating metal hydroxide electroplating sludges by fusion of the oxides of the metals into Silica and Sodium slag. The Rostoker method relates to earlier EP Leaching standards, and has been proven incapable of achieving minimal-waste recycling. Lynn, U.S. Pat. No. 4,840,671 issued Jun. 20, 1989, relates to the stabilization of EAF dusts for disposal. The latter '671 reference teaches the use of calcium hydroxides as an entrapping agent for toxic cadmium, chromium, and lead constituents. This patent suggests combining various different waste products to be processed to produce "safe" compounds.
However, in view of the general awareness of environmental and health risks, such treatment and disposal techniques are no longer deemed environmentally or economically sound. Moreover, because industry pays a relatively high premium for waste treatment and disposal, it is desirable to provide a commercially viable method for recovering as much usable material as possible. Such methods would be directed to reducing loss of profits and expanding commercial markets. Specifically, it is desired to provide a process that can be carried out with equipment and apparatus already available in the industry.
The United States Environmental Protection Agency (U.S. EPA) classifies certain materials for controlled disposal and/or recovery. The U.S. EPA has determined that any products that are made from a hazardous waste and sold within one year of production is not a hazardous waste but a product. This product must be of known commercial value. This product cannot be a replacement for any product that is typically used on the land in any form. If the product from any hazardous waste reuse, recycle or reclaim system does not meet all of the rules the product is not a product but a waste. These rules state that any byproduct or waste that is derived from a process using hazardous waste, is a hazardous waste, even when this waste contains nothing that was in the original hazardous waste. The list of U.S. EPA-listed hazardous wastes is presently limited, but will undoubtedly be enlarged with time. There are presently a large number of waste products generally recognized as unsafe for conventional disposal which have not yet come under U.S. EPA scrutiny. For example, certain anodizing wastes such as F019 are presently listed but not classified as hazardous; sand used in blasting operations may be contaminated with nickel, chrome, or other metals that are considered toxic; and, baghouse dusts may contain carbon and hazardous materials that have no separate classification under the current law. Such wastes are therefore thrown away without meaningful disposal precautions, although they are widely believed to create hazards to the environment. Moreover, their disposal results in unnecessary depletion of existing natural mineral resources.
It is therefore desired to provide a viable method for reclaiming various listed and non-listed hazardous wastes. Such a method must effectively eliminate waste to conserve natural resources and avoid costly liability, for example, under the Resource Conservation and Recovery Acts ("RCRA") or Comprehensive Environmental Response, Compensation, and Liability Act ("CERCLA"). Moreover, such a method must be effective to overcome disadvantages associated with prior sodium-based recovery processes, such as high reagent cost, sodium volatilization at higher temperatures, pH imbalances in slags, low value byproducts, and production of waste.
In the prior art known to us, numerous methods are taught for recovering various industry wastes for production of useful products. Such products include furnace fuels, paving aggregates, sealing compounds, and mineral wool.
Mineral wool is a term broadly applied to various related vitreous products commonly used for insulation, padding, ceiling tile production, and the like. In general, mineral wool is a fiberglass-like material composed of very fine, interlaced mineral fibers, somewhat similar in appearance to loose wool. It is composed primarily of silicates of calcium and aluminum, chromium, titanium, and zirconium. Mineral wool producers commonly use natural rock or slag. Slag is a term broadly applied to refer to waste products of the primary metal and foundry industries, including deposits from the furnace lining, charge impurities, ash from fuel, and fluxes used to clean the furnace and remove impurities. Although metal producers and foundries strive to control the amount of slag, excess slag may result from the refining of metals.
Slags are classified as either "acid" (or high silicate) slags or "basic" slags, depending upon the relative quantities of acidic and basic sub-components. For example, typical acid slags contain between forty and fifty percent (40-50%) of acidic sub-component, such as silicon (SiO.sub.2) and relatively small quantities ranging from one to five percent (1-5%) of basic sub-components, such as oxides of calcium (CaO) and magnesium (MgO). A typical basic slag that is used to refine or reduce metals comprises between twenty-five and fifty percent (25-50%) acidic sub-components such as silicon (SiO.sub.2) and aluminum (Al.sub.2 O.sub.3), and a relatively high percentage, from thirty-four to fifty percent (34-50%) basic subcomponents, such as oxides of calcium (CaO) and magnesium (MgO). Magnesium may be added to increase basicity. Basicity is the tool used to determine metal quality using basic slag. Basicity is calculated as follows: CaO+MgO/Al.sub.2 O.sub.3 +SiO.sub.2. Basicity of typical basic slags ranges between 0.93 and 1.9.
Typically metal producers are most interested in the highest quality metals with the least amount of slag production. Traditionally it is expensive to melt slag, and the slag is of little worth or it is an environmental liability. Because high quality scrap metal is abundant, modern metal producers prefer to use acid slags and not refine metals. For best results, mineral wool producers seek slags or rock that can be blended together and melted at relatively low temperatures. Preferably the mineral wool slag will contain no reducible metals, or will be an acid slag that will eliminate metal buildup in the furnace.
Mineral wool is classified according to the raw materials used in its production. For example, Rock Wool is produced from combinations of natural rocks and/or minerals. Slag Wool comprises a composition of iron, copper, and lead slags typically removed from blast furnaces, and may contain some fluxing materials. Glass Wool (fiberglass) is composed principally of silica sand, soda ash, and limestone. Refractory (high-temperature) or "Certa" wool may be made from oxides of aluminum, chromium, zirconium, or titanium and silica sand. Further subclassifications of these products relate to the quality or purity of the wool. For example, slag wool is subclassified for purity according to color; black, gray, and white wools are available. The tool for determining the quality of mineral wool produced from a slag charge is the Acid-to-Base ratio (A:B). The formula for determining A:B is Al.sub.2 O.sub.3 +SiO.sub.2 /CaO+MgO. In a typical mineral wool cupola slag, A:B ranges between 0.74 and 2.316.
Prior art patents related to the production of mineral wool using various waste products include Gee U.S. Pat. No. 4,822,388 issued Apr. 18, 1989; and U.S. Pat. No. 4,486,211 issued to Monaghan on Dec. 4, 1984. The latter-referenced '211 patent discloses a method and apparatus for melting discarded fly ash and spinning it into mineral wool. However, none of the prior art known to us teaches viable methods for recycling listed hazardous materials containing waste metal oxides such as chromium, nickel, cadmium, zinc, copper, iron, and lead oxides or hydroxides into pure metals or alloys while producing mineral wools from aluminum, silica, calcium, zirconium, and titanium oxides.
Other relevant prior art patents known to us relate to methods for treatment, recovery, and recycling. For example, Allen, U.S. Pat. No. 3,870,507, issued Mar. 11, 1975 is directed to a method for forming briquettes from steel mill wastes such as steel and iron dust, mill scale, and iron oxides with an organic binder to reduce slags formed during recycling. The resulting iron oxide briquettes are recycled by being fed into the production furnaces with new materials in the steel-making process.
U.S. Pat. No. 4,004,918 issued to Fukuoka on Jan. 25, 1977, teaches a method for treating certain wastes resulting from stainless steel operations. Briquettes are formed from the dust and scale from stainless steel ovens combined with organic and inorganic binders. The briquettes are returned to the existing electric arc furnace, and usable metals are extracted for further use in making stainless steel.
Stephens U.S. Pat. No. 4,396,423 issued Aug. 2, 1983 and related U.S. Pat. No. 4,053,301 issued October, 1977 relate to a process for recovery of iron carbide and zinc metals from BOF dusts of the steel-making process. The Stephen's system reduces the dust wastes within a fluidized bed reactor in the presence of carbon, recovers zinc by vaporization, and produces iron carbide and gangue, a worthless rock or matter in which metals are contained.
U.S. Pat. Nos. 4,758,268 issued Jul. 19, 1988 and 4,836,847 issued Jun. 6, 1989 to Bishop disclose apparatus and processes for reclaiming metals from electric arc furnace and BOF dusts. The systems described therein are directed to providing recovery of metals from EAF wastes in a reducing environment. In the method, carbon is added to the molded briquettes to reduce the iron and zinc content of the waste. However, the process is incapable of producing a slag suitable for use in the production of mineral wool, since these processes attempt to minimize slags to less than 8%. Moreover, the Bishop system is specifically indicated to be unsuited for rotary kilns, shaft furnaces, retorts, and fluidized bed furnaces.